1. Field of the Invention
The invention relates to cordierite ceramic bodies for use as catalyst carriers, particularly to cordierite bodies, having ultra-thin web sizes, for use as catalyst carriers for purifying automobile exhaust gas, and particularly to a method for producing these ultra thinwall cordierite structures.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of pollutants from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970""s.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to cordierite ceramics"" high thermal shock resistance. The production of cordierite (2MgO.2Al2O3.5SiO2) ceramics from mineral batches containing sources of magnesium, aluminum and silicon such as clay and talc is well known. Such a process is described in U.S. Pat. No. 2,684,919. U.S. Pat. No. 3,885,977 discloses the manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low expansion coefficients along at least one axis.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to develop cordierite honeycomb substrates that exhibit high geometric surface area, which in turn leads to increased emission conversion efficiency and reduced precious metal catalyst loading. One means for achieving this increased surface area is too create substrates exhibiting a higher cell density. Demand for cordierite monoliths having increased geometric surface area (i.e., higher cell densities) is increasing in response to legislation requiring higher conversion efficiencies in catalytic converters for the automobile market.
One means to achieve the production of ceramic honeycombs with higher cell densities and increased geometric surface areas is the use of more sophisticated, higher cell density die designs capable of producing these higher cell density substrates. Specifically, these aforementioned more sophisticated die designs exhibit a discharge slot array comprising a very large number of very fine slots; i.e., higher and higher slot densities and therefore more densely packed and smaller feedholes.
The principal difficulty encountered with these slot arrangements is that there is a practical minimum feedhole size, due principally to drilling technology limitations, which limits the density of the feedhole patterns available. Thus, even at minimum attainable feedhole sizes, a too close spacing of feedholes produces a weak die structure. Also the smaller feedhole sizes increase the flow impedance of the die necessitating higher extrusion pressures. In general, then, a feedhole pattern permitting the use of smaller and/or more densely packed feedholes provides both die fabrication and die performance disadvantages.
In addition to the practical difficulty of reduced die feedhole sizes leading to weakened dies, the production of honeycomb substrates with very high cell densities/ high geometric surface areas is very difficult, when compared with substrates with lower cell density having a more conventional geometry. It has been found that when conventional high slot density extrusion apparatus are used to produce ceramic honeycombs with cell density exceeding 500 cells per in2, an unacceptably high number of breaks in the web of the cellular extrudate (i.e., areas containing no ceramic material) are observed in the extruded product. It is thought that these breaks in the ceramic material result from one or more particles from the extrusion material plugging a slots in the extrusion die, resulting in a region where batch is restricted from flowing. The number of breaks increases as the slot width decreases, and if the slot width is narrow enough, the number of plugged cells becomes so great that the extrudate does not hold together, but rather the extrusion consists of many small strands of batch material.
An additional means for producing increased cell density substrates includes the development of multi-component dies. Although multi-component die designs solves the limitations of conventional one-piece die design, specifically the weakness of die with narrow slots/feedholes and difficulty or inability to drill very small feedholes/slots, these multi-component die designs are extremely complicated and expensive to produce.
The discovery of a method of higher cell density, increased surface area ceramic honeycombs that overcome the aforementioned shortcomings of conventional methods and a method that is capable of being used with conventional extrusion apparatus would be highly desirable and an advancement in the art.
It is therefore a principal object of the present invention to provide an improved method for making high cell density, ceramic bodies, that produces little, if any discontinuities in the ceramic article and which can be utilized with conventional extrusion apparatus/dies. In spite of prior art that that suggests that high shrinkage should be avoided in the formation of ceramic honeycomb bodies, it has been found that when certain combinations of raw materials are used, the raw materials upon firing shrink so as to result in the radial shrinkage of the honeycomb body. Specifically, it has been discovered that certain combination of fine and high surface area raw materials, e.g., talc, clay and alumina for cordierite, when used in the preparation of ceramic honeycomb structures, form, upon extrusion a green body that will be subject to greater than 9% radial shrinkage upon subsequent firing.
More specifically, this invention relates to a method of producing a honeycomb ceramic body exhibiting a predetermined radial dimension comprising producing a green ceramic honeycomb body that exhibits a radial dimension at least 9% greater than the predetermined radial dimension and a cell density of at least 500 cpsi. The method further involves shrinking the green body during firing to form a sintered honeycomb ceramic body exhibiting the final predetermined radial dimension.
This invention also relates to a method of producing a ceramic body comprising the following steps:
(a) compounding and plasticizing a ceramic raw material mixture and forming the plasticizable raw material mixture into a green ceramic body by extrusion through an extrusion die;
(b) drying the green body and thereafter firing the green body at a time and at a temperature sufficient to sinter the ceramic body resulting in a radial shrinkage of the green ceramic body in the radial dimension due to the firing of at least 9%.
The advantage of the aforementioned methods is that the utilization of these methods involving a high degree of radial shrinkage is used, allows the formation of increased cell density honeycomb bodies the use of a conventional and much less expensive die design is made possible rather than the use of a combination of a multi-component die and low level of shrinkage-materials.